Noob Question RE Capacitor Types in Tube Amp Circuits

[TraitorJoe]

Hello! Total noob here, hoping for some advice regarding tube amp circuits, if you please.

TL/DR - Can I buy a bunch of cheap ceramic capacitors and use them in all circuit locations except for filtering stages? Why/why not?

Details:

I see that caps of identical "ratings" (capacitance/voltage) are constructed differently (ceramic VS metallized polyester, etc.)

As far as I understand, electrolytic caps are called for in certain circuit locations for good reasons.

However, I can't seem to find out why one might use a capacitor of one type of construction vs a different type of construction in any other location within a circuit.

Assuming matched capacitance & voltage ratings, what's stopping me from just using cheap ceramic caps in all circuit locations except for filter caps? Is this a tone thing? Heat?

Come to think of it, is it possible to accomplish everything with ceramic caps, including filtering?

Thank you for your time and attention.

[WimWalther]

Very briefly,

Ceramic caps aren't made in values large enough for power supply filters, which are typically 20-50uF per cap or section. This is why electrolytic types are used in these locations.

Other than the basic ratings of value & voltage, there are a number of other cap characteristics that make certain types preferred or required in various locations. These are parameters like ESR, dissipation factor, temperature coefficient, frequency response and so forth.

And then.. there's the whole "tone" debacle. Because it turns out that various types of cap tend to sound differently from others, and this leads to preferences for one over another. It may go without saying, that this can be a deep rabbit hole.

[WimWalther]

As to ceramics used as coupling caps in tube amplifiers, they are almost universally looked down upon as unsuitable..

But, fwiw, there is a very highly-regarded vintage integrated amp called an Eico HF-81. These units use bog-standard ceramic caps for output stage coupling, which is generally considered a sensitive location. Cheap ceramics or nor, these amps are thoroughly musical and very enjoyable as-is.

(Some of their great sound is surely due to their use of EL84 / 6BQ5 power tubes. This is a very musical tube type, my own personal favorite of the pentodes / beam-power tubes, and I've never met an EL84 amp that I didn't like.)

Feel free to experiment. Get a few caps of different manufacture & type, and spend some time auditioning them. You'll probably learn a bit.

[TraitorJoe]

Thank you, WimWalther, I will certainly do some experimenting.

I'm also a fan of EL84's. I assembled a marshall 18 watt clone (BabyWill) a few years ago and I love it.

I need more volume however so I'm planning on building an AC30 clone. I also want to see how a tube rectifier sounds/feels; my BabyWill is solid-state rectified and so are the other tube amps I've tried/owned.

Thanks again,

[Esquirefreak]

My experience with ceramic caps is that when larger nF values are needed (like >4.7nF) they tend to be microphonic. But for instance in a tremolo section or where the audio signal doesn't run it matters less. For caps in the pF range, I usually prefer ceramics, due to the lower price.

/Max

[tdvt]

An interesting video, especially how temp-sensitive things are.


Apparently tube amps already use less-than-ideal components in various places.

[sluckey]

[HotBluePlates]

... I see that caps of identical "ratings" (capacitance/voltage) are constructed differently (ceramic VS metallized polyester, etc.)
...
However, I can't seem to find out why one might use a capacitor of one type of construction vs a different type of construction in any other location within a circuit.

... what's stopping me from just using cheap ceramic caps in all circuit locations except for filter caps? ...

The dominant factor in "construction type" is the amount of capacitance and/or voltage rating.  Both affect the physical size of the finished capacitor, and juggling capacitance/voltage/size then results in certain types being available for certain µFs/volts.



Everything else about the goodness/badness of this dielectric or that is mostly a distraction.

[WimWalther]

@sluckey

You know what I think about that pic? I think you should remove two of those 1/4" TS panel jacks w/switch and send them to me.

Seriously. I've ordered jacks for my Hammond / 6G2 projects twice, now, and neither shipment ever arrived. Still got nothin'.

Come to think, I'll need 4 total, eventually. For real, somebody please send / sell me some of those.. used is great.

[sluckey]

Seriously. I've ordered jacks for my Hammond / 6G2 projects twice, now, and neither shipment ever arrived. Still got nothin'.

If you had ordered from Hoffman on Wednesday you would have them installed by now.   

[PRR]

....As far as I understand, electrolytic caps are called for in certain circuit locations for good reasons.

The main "good reason" is CHEAP. Over a wide range of V/uFd, electros run 1/10th the price of any other.

Remember (or notice) that we can buy capacitors over a v-e-r-y wide range of values, wider than any other common part. 5pFd to 5Farad is a million-million to one. Resistors come off the shelf only about 0.1r to 10M or 100M, say a thousand million range. There has to be an enormous spread of materials and constructions to do that economically. All generalizations are false (including this one).

When you budget for electrolytics you have to ponder their significant faults. Uncertain value. Leakage, both electrons and sometimes goo. Not infinite life, and sometimes very short life.

Ceramics below 1000pFd can be pretty perfect. Over 0.1uFd they can be kinda bogus. Capacitance varying a LOT with voltage (bass distortion) and temperature.

[WimWalther]

I need more volume however so I'm planning on building an AC30 clone.

If your 18W amp is too low on volume, going to a ~30W amp may not give you what you're looking for.

Doubling power is really a bare minimum when in need of more volume. A fourfold increase is usually what you'd want for a really satisfying boost.

I'd look at building something in the 65-75W range.. just my advice. Going to a cabinet with more, and more sensitive drivers could also be a good route.

[glass54]

TraitorJoe,
Leo used them (Ceramics) in many places including PI coupling.
Regards
Mirek

[WimWalther]

TraitorJoe,
Leo used them (Ceramics) in many places including PI coupling.

I don't doubt it. I've heard them sound fine, as in the HF-81, and I've heard them sound awful, as in bizarrely broken, in my own experimentation.

Another fine sounding but "cheap" and otherwise very average film cap is the Illinois MWR. These work great in old radios and should be as good as any of the vintage types in guitar amps.

The Illinois MPW is a similar part, but they're not as sonically nice as the MWR in my experience.. though fairly close.

[glass54]

Just to clarify:

Leo used them

Referring back to reply 12 In Tone shaping Leo used quality caps BUT in less critical places he used Ceramics (He was trying to save $/scrimped a little and some may even suggest he was a bit of a cheapskate  ()
History tells us he got a reasonable balance.
Regards
Mirek

[WimWalther]

@glass54

Any sense of the difference in prices between equivalent ceramic & paper / foil caps back in Leo's day?

Ceramics tend toward lower values and higher voltages, but there must be some comparisons to be drawn, I'd think?

[HotBluePlates]

... In Tone shaping Leo used quality caps BUT in less critical places he used Ceramics (He was trying to save $/scrimped a little and some may even suggest he was a bit of a cheapskate  ( ) ...

Guitarists wrongly assume everything picked in an amp is about "Tone."

I notice that Fender used ceramic disk caps & "brown blob caps" in 1960s amps in locations on the fiberboard where they needed close-spacing of the legs.  Where wider spacing was allowable, Fender used the blue molded caps.  Some of those ceramics were for the tremolo oscillator, but the "brown blob" was a coupling cap.

I think Fender selected whatever part was economical & fit the space allowed. 

[acheld]

Oh lord, this is THE rabbit hole . . .

We have so many choices now as compared to "back in the day."   Ceramic caps really vary (a lot) in their response to heat, and microphony.  You really have to look at the specs closely.  For example, Class I (such as C0G or NP0) are cap types that are quite stable and appropriate for guitar tube amps.   Class II less so (as a general rule), but there are so many variations that you can't generalize easily.

And then, why for our filter caps do we always revert to electrolytics?  Film caps can do the job with higher ripple tolerance, better heat tolerance and longevity (but are larger) than 'lytics.

Coupling caps vary in both the dielectric used and construction.   CDE orange drops definitely have a foil "out" side for your lower impedance  connection.   But many of the newer construction film caps seem to have no significant difference when I check them for polarity.

It's just a different world now from the days of the blue molded and my favorite (the dreaded) Astron cap.

[WimWalther]

And then, why for our filter caps do we always revert to electrolytics?  Film caps can do the job with higher ripple tolerance, better heat tolerance and longevity (but are larger) than 'lytics.

The only film caps I'm aware of that are large enough (value & volts) to sub for lyrics are the parts made by Solen and AEON. And those are both expensive, boutique lines. This ignores oil-filled bathtub types.

Do you know of any reasonable alternatives?

[acheld]

The only film caps I'm aware of that are large enough (value & volts) to sub for lyrics are the parts made by Solen and AEON. And those are both expensive, boutique lines. This ignores oil-filled bathtub types.

Do you know of any reasonable alternatives?

   Reasonable, I don't know.   But I have been using components like this for the past three years or so with good luck.  Developed primarily for automotive applications, these puppies are tough.   

I mount them in a doghouse. 

https://www.mouser.com/datasheet/2/447/KEM_F3116_C4AF-3316616.pdf  or  https://www.mouser.com/datasheet/2/315/Panasonic_6282023_EZPV_Series_120122-3224020.pdf

[WimWalther]

Guitarists wrongly assume everything picked in an amp is about "Tone."

Just as both musicians and audiophiles wrongly assume that any "military" (JAN spec, etc) tube is the superior device for their particular application, whatever that may be.

The military specified tubes that were  controlled & optimized for a variety of parameters, but with the possible exception of noise & microphonics, their "sonic merits" were not among them.

If a milspec type sounds subjectively "better" than commercial production, it's more a matter of coincidence than intention.

[PRR]

> Do you know of any reasonable alternatives?

Acheld suggests a line of Kemet caps. Taking "40u 450V" as a useful tube-amp cap, we find:

C4AF1EW5470A3AK   Film Capacitor  250V(AC, 500VDC) 47 uF 105C 10%  -  $17.33

Hoffman will sell you a:
47uf/500v F & T brand electrolytic capacitor = $6.90
--almost a third the price.

If you figure 15 year life on the e-cap, after 45 years you come out ahead. Oh, wait, Hoffman wants his cash now for long-distant performance, so add 1% to 9% interest (present cost of future money). OTOH two e-cap replacements is a labor expense.

If you aren't the telephone company, film caps are hard to justify on economics. For the same bucks, you can buy many more uFd of e-cap which in most audio "is better" even for sound. (I used to pack Bogen PAs with e-caps to good effect.)

OTOOH, after you mop-out a pint of yuck from rotten e-caps, the film caps do look good.

[acheld]

Acheld suggests a line of Kemet caps.

In my defense, my post began with:  "   Reasonable, I don't know.  "

The two film caps I provided data sheets for are quite stout with very good ripple tolerance and low ESR.  They can be self healing with regard to overvoltage conditions.  They tolerate heat well.   Their major drawbacks are their size and their cost.  I DO use these in the power supply;  I can tuck them in a doghouse (see photo). 

It makes no sense to use film caps in other positions where their other attributes are real disadvantages - cathode bypass caps for example (film caps are too big, there is no need for high voltage protection, and radial electrolytics can be spec'd to sufficient heat tolerance and long life).

Film caps are indeed more expensive.   This is a real issue for production amps; not so much for a hobbyist. 

I am not recommending these caps as being "better."   They are better in some applications.  Whether guitar amps is one of those applications is debatable. 

But yeah, I like them in the power supply.

[sluckey]

Some guys over at TAG have used film caps for filter caps.

[mresistor]

Just Radios in Canada is another source of hard to find film capacitors in larger size and ratings..      scroll down  on left side   
 
 
https://www.justradios.com/cart.html


he also has x1/y2,  x  and y  safety capacitors

[PRR]

> In my defense, my post began with:  "   Reasonable, I don't know.  "

I shouldn't have made you defensive. I am sorry.

I also admit to sometimes paying "more", and for even less reason. (See today's $27 lunch, most of which I didn't even bring home for the dogs.)

The hi-fi guys sometimes like film caps as power filters because they think their stuff don't stink and will live forever.